Rubber composition and pneumatic tire using the same in tread

ABSTRACT

A rubber composition comprising as a rubber component a specified styrene-butadiene copolymer (A) and a styrene-isoprene copolymer of a specified hydrogenation ratio (B) wherein a bound styrene content of the copolymer (B) is larger by a given value than a bound styrene content of the copolymer (A) is good in the wear resistance and fracture property and further excellent in the wet-skid resistance and dry gripping property.

TECHNICAL FIELD

This invention relates to a rubber composition suitable for a tread of ahigh-performance pneumatic tire, and more particularly to a rubbercomposition suitable as a tread rubber of a pneumatic tire having goodwear resistance and fracture properties and a further improved grippingproperty.

BACKGROUND ART

Recently, the demand on the high-speed running stability of the vehiclebecomes further severer, and hence it is strongly desired to use rubberhaving excellent wet-skid resistance and dry gripping property inaddition to the wear resistance and fracture properties required fromthe past as a rubber material for a tire tread in the vehicle. However,it is difficult to simultaneously satisfy these properties, andparticularly the wear resistance, wet-skid resistance and dry grippingproperty have a conflicting relation to each other. The grippingproperty is dependent on the hysteresis loss of the rubber composition.In order to enhance the gripping property of the rubber composition, agreater amount of an aromatic type softening agent has hitherto beencompounded, which brings about the lowering of the fracture properties.

In order to solve this problem, the invention disclosed inJP-A-63-101440 proposes the use of low molecular weight SBR. However, adouble bond having a crosslinking property is existent though themolecular weight is low, so that there is a problem that a part of thelow molecular weight component is crosslinked with rubber matrix andtaken into the matrix and hence the sufficient hysteresis loss is notproduced. Also, if the double bond part is rendered into a saturatedbond by hydrogenation so as not to take the low molecular weightcomponent into the matrix through the crosslinking, the compatibilitywith the matrix becomes considerably poor, and as a result, there areproblems that the fracture properties lower and the low molecular weightcomponent bleeds out.

Furthermore, rubber compositions aiming at the improvement ofworkability, low fuel consumption, fracture properties and grippingforce by a blend of high molecular weight body and a low molecularweight body are disclosed in the inventions of JP-B-59-52664,JP-A-58-147442, JP-A-58-147443, JP-A-60-240746, JP-A-61-203145,JP-A-62-135506, JP-A-64-16845 and the like, but they can notsufficiently satisfy the wet-skid resistance, dry gripping property andwear resistance.

JP-A-2000-129037 discloses a technique solving the above problems. Thistechnique uses a rubber composition using a specified styrene-butadienecopolymer (A′) and a styrene-butadiene copolymer having a specifiedhydrogenation ratio (B′) as a rubber component, in which a bound styrenecontent of the copolymer (B′) is made larger by a given value than abound styrene content of the copolymer (A′), as a rubber material for atire tread. However, tires having a further improved gripping propertyare demanded at the present, and the gripping property is insufficienteven in the tire using the rubber composition disclosed in theJP-A=2000-129037 in the tread.

DISCLOSURE OF THE INVENTION

It is an object of the invention to provide a novel rubber compositioncapable of further improving the wet-skid resistance and dry grippingproperty in addition to good wear resistance and fracture properties forfurther improving the gripping property in the high-speed running of thevehicle as well as a pneumatic tire using this rubber composition as atread rubber.

The inventors have found that a hydrogenated styrene-isoprene copolymerhaving a specific microstructure is high in the tackiness as comparedwith the conventional hydrogenated styrene-butadiene copolymer and thewear resistance, fracture properties, wet-skid resistance and drygripping property can be further improved by using the hydrogenatedstyrene-isoprene copolymer instead of the hydrogenated styrene-butadienecopolymer, and as a result, the invention has been accomplished.

That is, the rubber composition of the invention comprises a rubbercomponent formed by compounding 100 parts by mass of styrene-butadienecopolymer (A) polymerized with a lithium based polymerization initiator,in which a polystyrene-conversion weight average molecular weightthrough a gel permeation chromatography is 4.0×10⁵-3.0×10⁶ and a boundstyrene content is 10-50 mass % and a vinyl bond content in butadieneportion is 20-70%, with 10-200 parts by mass of a hydrogenatedstyrene-isoprene copolymer (B), in which a polystyrene-conversion weightaverage molecular weight through a gel permeation chromatography is5.0×10³-2.0×10⁵ and a bound styrene content is 25-70 mass % and not lessthan 60% of double bond in isoprene portion is hydrogenated, and havinga relation between the bound styrene content of the polymer (A) and thebound styrene content of the polymer (B) satisfying the followingequation:Bound styrene content of polymer (B)>bound styrene content of polymer(A)+10 (mass %).

In a preferable embodiment of the invention, the polystyrene-conversionweight average molecular weight of the copolymer (A) is 7.0×10⁵-2.5×10⁶.

In another preferable embodiment of the invention, the bound styrenecontent of the copolymer (A) is 20-40 mass %.

In the other preferable embodiment of the invention, the vinyl bondcontent of the butadiene portion in the copolymer (A) is 30-60%.

In a further preferable embodiment of the invention, not less than 80%of double bond in the isoprene portion of the copolymer (B) ishydrogenated.

In a still further preferable embodiment of the invention, the boundstyrene content of the copolymer (A) and the bound styrene content ofthe copolymer (B) satisfy a relation of the following equation:Bound styrene content of polymer (B)>bound styrene content of polymer(A)+15 (mass %).

In a further preferable embodiment of the invention, the copolymer (B)is compounded in an amount of 20-100 parts by mass based on 100 parts bymass of the copolymer (A).

Also, the pneumatic tire according to the invention is characterized byusing the above rubber composition as a tread rubber.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be described in detail below. In the rubbercomposition of the invention, the styrene-butadiene copolymer (A) havingspecified ranges of polystyrene-conversion weight average molecularweight, bound styrene content and vinyl bond content in butadieneportion and the styrene-isoprene copolymer (B) having specified rangesof polystyrene-conversion weight average molecular weight, bound styrenecontent and hydrogenation ratio of double bond in isoprene portion areused as a rubber component, in which the bound styrene content of thecopolymer (B) is made larger by a given value than the bound styrenecontent of the copolymer (A). In the invention, by using thestyrene-isoprene copolymer (B) instead of the styrene-butadienecopolymer (B′) constituting the rubber composition of the aforementionedJP-A-2000-129037 are further improved the wear resistance, fractureproperties, wet-skid resistance and dry gripping property in the tirefor the vehicle because the styrene-isoprene copolymer (B) is high inthe tackiness as compared with the conventional styrene-butadienecopolymer (B′).

In the invention, the polystyrene-conversion weight average molecularweight obtained through a gel permeation chromatography (GPC) of thecopolymer (A) is defined to be 4.0×10⁵-3.0×10⁶. When it is less than4.0×10⁵, the fracture properties lower, while when it exceeds 3.0×10⁶,the viscosity of the polymer solution becomes too high and theproductivity lowers. From the same viewpoint, it is preferably7.0×10⁵-2.5×10⁶.

Also, the bound styrene content of the copolymer (A) is defined to be10-50 mass %. When it is less than 10 mass %, the fracture propertieslower and it is difficult to simultaneously satisfy the wet-skidresistance and other properties, while when it exceeds 50 mass %, thewear resistance lowers. From the same viewpoint, it is preferably 20-40mass %. Furthermore, the vinyl bond content in butadiene portion of thecopolymer (A) is defined to be 20-70%. When it is less than 20%, thewet-skid resistance is insufficient, while when it exceeds 70%, the wearresistance lowers. From the same viewpoint, it is preferably 30-60%.When the bound styrene content and vinyl bond content in butadieneportion of the copolymer (A) are within the above preferable ranges, thebalance between wet-skid resistance and the wear resistance isexcellent.

In the invention, the polystyrene-conversion weight average molecularweight obtained through GPC of the copolymer (B) is defined to be5.0×10³-2.0×10⁵. When it is less than 5.0×10³, the fracture properties,wear resistance, wet-skid resistance and dry gripping property are poor,while when it exceeds 2.0×10⁵, the wet-skid resistance and dry grippingproperty are poor.

Also, the bound styrene content of the copolymer (B) is defined to be25-70 mass %. When it is less than 25 mass %, the wet-skid resistanceand dry gripping property are insufficient, while when it exceeds 70mass %, the copolymer is resinified to make the composition hard and thewet-skid resistance and dry gripping property lower.

Furthermore, it is defined that not less than 60% of double bond inisoprene portion of the copolymer (B) is hydrogenated. When thehydrogenation ratio is less than 60%, co-crosslinking with the copolymer(A) is caused and hence the sufficient gripping property is notobtained. As the hydrogenation ratio becomes higher, the co-crosslinkingproperty with the copolymer (A) lowers and hence the hysteresis lossbecomes high and the excellent gripping property is developed. From thisviewpoint, a preferable range of the hydrogenation ratio is not lessthan 80%.

In addition, the rubber composition of the invention defines that thecopolymer (B) is compounded in an amount of 10-200 parts by mass basedon 100 parts by mass of the copolymer (A). When the amount is less than10 parts by mass, the improvement of the wet-skid resistance and drygripping property is insufficient, while when it exceeds 200 parts bymass, the Mooney viscosity is too low and the productivity becomes poor.From the same viewpoint, it is preferably 20-100 parts by mass.

Moreover, the rubber composition of the invention defines that the boundstyrene content of the copolymer (A) and the bound styrene content ofthe copolymer (B) satisfy the relation of the following equation:Bound styrene content of polymer (B)>bound styrene content of polymer(A)+10 (mass %),which is required to obtain the compatibility between the copolymer (A)and the copolymer (B). When the difference of the bound styrene contentis not more than 10 mass %, the compatibility is not obtained and thebleeding of the copolymer (B) toward the rubber surface is caused, andhence when it is used as a tread rubber of the tire, the sufficientadhesion to the other members such as casing rubber and the like is notobtained and also the sufficient strength at break is not obtained. Byusing the copolymer (B) satisfying the above condition can be obtainedthe rubber composition satisfying the excellent strength and grippingproperty. In order to obtain the more complete compatibility, it ispreferable that the difference of bound styrene content between thecopolymer (A) and the hydrogenated copolymer (B) is not less than 15mass %.

The copolymer (A) is obtained by copolymerizing butadiene and styrene ina hydrocarbon solvent in the presence of ether or a tertiary amine witha lithium based polymerization initiator.

As the hydrocarbon solvent can be used an alycyclic hydro-carbon such ascyclohexane, methyl cyclopentane, cyclooctane or the like; an aliphatichydrocarbon such as propane, butane, pentane, hexane, heptane, octane,decane or the like; and an aromatic hydrocarbon such as benzene,toluene, ethylbenzene or the like. These hydrocarbons may be used aloneor in a combination of two or more. Among these hydrocarbons, thealiphatic hydroxarbon and alicyclic hydrocarbon are preferable.

As the polymerization initiator is preferable an organolithium compound,which includes an alkyllithium such as ethyllithium, propyllithium,n-butyllithium, sec-butyllithium, tert-butyllithium or the like; anaryllithium such as phenyllithium, tolyllithium or the like; analkenyllithium such as vinyllithium, propenyllithium or the like; analkylene dilithium such as tetramethylene dilithium, pentamethylenedilithium, hexamethylene dilithium, decamethylene dilithium or the like;an arylene dilithium such as 1,3-dilithiobenzene, 1,4-dilithiobenzene orthe like; 1,3,5-trilithiocyclohexane, 1,2,5-trilithionaphthalene,1,3,5,8-tetralithiodecane, 1,2,3,5-tetralithio-4-hexylanthracene and thelike. Among them, n-butyllithium, sec-butyllithium, tert-butyllithiumand tetramethylene dilithium are preferable, and n-butyllithium isparticularly preferable.

The amount of the organolithium compound used is determined by apolymerization rate in the reaction operation and a molecular weight ofthe resulting polymer, but it is usually about 0.02-5 mg, preferably0.05-2 mg as a lithium atom per 100 g of a monomer.

The polymerization reaction for obtaining the copolymer (A) may becarried out by any one of a batch polymerization system and a continuouspolymerization system. The polymerization temperature in the abovepolymerization reaction is preferable to be within a range of 0-130° C.Also, the polymerization reaction may be conducted by any polymerizationtypes such as isothermal polymerization, temperature rise polymerizationand adiabatic polymerization. Further, an allene compound such as1,2-butadiene or the like may be added for preventing the formation ofgel in a reaction vessel during the polymerization.

On the other hand, the hydrogenated styrene-isoprene copolymer (B) canbe obtained by hydrogenating a polymer synthesized in the same manner asin the copolymer (A) except that isoprene is used instead of butadienethrough the usual hydrogenation method. That is, the hydrogenation iscarried out under a pressurized hydrogen of 1-100 atmospheric pressureby using a catalyst selected from a hydrogenation catalyst such as anorganic carboxylic acid nickel, an organic carboxylic acid cobalt andorganometallic compounds of Group I-III; a catalyst of nickel, platinum,palladium, ruthenium or rhodium metal carried on carbon, silica,diatomaceous earth or the like; a complex of cobalt, nickel, rhodium orruthenium; and so on.

In the invention, the polymerization solution of the copolymer (B) isadded to the polymerization solution of the copolymer (A) to obtain apolymerization reaction solution containing styrene-butadiene copolymerand styrene-isoprene copolymer, and rubber component and the solvent areisolated from this solution by a method used in the usual solutionpolymerization method (for example, a method wherein a stabilizing agentor the like is added at a solution state and then directly dried orsubjected to steam stripping) and washed and dried to obtain the rubbercomposition according to the invention.

The rubber component in the rubber composition of the inventioncomprises only the copolymer (A) and the copolymer (B), or may beblended with the other diene rubber(s) such as natural rubber,polyisoprene rubber, emulsion polymerized styrene-butadiene rubber,polybutadiene rubber or the like. Such a rubber component is added witha reinforcing agent such as carbon black, silica or the like, andvarious additives and milled in a roll, a Banbury mixer, a kneader orthe like and then added with sulfur, a vulcanization accelerator and thelike, which can be vulcanized to use as a tread rubber for the tire. Thecase that the rubber composition of the invention is blended with theemulsion polymerized styrene-butadiene rubber is particularly preferablefor use in a tire taking account of high-speed running, and the case ofblending with polybutadiene is particularly preferable for use in a tiretaking account of low-temperature characteristics.

The following examples are given in illustration of the invention andare not intended as limitations thereof. In these examples, variousproperties are evaluated by the following methods.

EXAMPLES

(1) The molecular weight (M_(w)) and molecular weight distribution(M_(w)/M_(n)) are measured under the following conditions by using 244model GPC made by Waters Corp. and a differential refractometer as adetector.

column: GMH-3, GMH-6, G6000H-6 columns made by Toyo Soda Co., Ltd.

mobile phase: tetrahydrofuran

(2) Polystyrene-converted weight average molecular weight

The polystyrene-converted molecular weight of the polymer is measured byusing a calibration curve which is previously prepared by using amonodisperse styrene polymer made by Waters Corp. and determining arelation between molecular weight of peak of the monodisperse styrenepolymer through GPC and count number of GPC.

(3) Vulcanization Properties

{circle over (1)} The strength at break is measured according to JISK6301.

{circle over (2)} The wet-skid resistance and dry-skid resistance areevaluated through a skid tester by duplicating wet road surface and dryroad surface.

{circle over (3)} The wear resistance is evaluated by a Lambournabrasion tester.

Synthesis of Copolymer (A-1):

In an autoclave of 5 liters sufficiently purged with nitrogen andprovided with a stirring blade are charged 3000 g of cyclohexane, 12 gof tetrahydrofuran (THF), 200 g of 1,3-butadiene and 100 g of styrene,and a temperature inside the autoclave is adjusted to 21° C. Then, 0.10g of n-butyllithium is added to conduct polymerization under atemperature rising condition for 60 minutes, and the conversion of themonomer is confirmed to be 99%. Thereafter, 3.5 g of2,6-do-t-butyl-p-cresol is added as an antioxidant. The analyticalvalues are shown in Table 1.

Synthesis of Copolymers (A-2)-(A-6):

The synthesis is conducted in the same manner as mentioned above exceptthat the charging ratio of monomers and the amount of the catalyst arechanged. The analytical results are shown in Table 1. TABLE 1 Copolymer(A) A-1 A-2 A-3 A-4 A-5 A-6 Bound styrene 33  8 33 20 33 41 content(mass %) Vinyl bond 40 40 40 60 80 35 content (%) Weight average 7.0 ×10⁵ 7.0 × 10⁵ 3.1 × 10⁵ 1.05 × 10⁶ 7.1 × 10⁵ 6.5 × 10⁵ molecular weightSynthesis of Copolymer (B-1):

In an autoclave of 5 liters sufficiently purged with nitrogen andprovided with a stirring blade are charged 3000 g of cyclohexane, 12 gof tetrahydrofuran (THF), 150 g of 1,3-butadiene and 150 g of styrene,and a temperature inside the autoclave is adjusted to 21° C. Then, 1.50g of n-butyllithium is added to conduct polymerization under atemperature rising condition for 60 minutes, and the conversion of themonomer is confirmed to be 99%. Thereafter, 4.68 g of tributylsilylchloride is added to stop the polymerization, and a catalyst solution ofnickel naphthenate:triethylaluminum:butadiene=1:3:3 (mol ratio)previously prepared in another vessel is charged so as to become 1 molof nickel per 1000 mols of butadiene portion in the copolymer.Thereafter, hydrogen is introduced into the reaction system under ahydrogen pressure of 30 atm to conduct the reaction at 80° C. Thehydrogenation ratio is calculated from a reduction of an unsaturatedbond portion in a spectrum of 100 MHz proton NMR measured at aconcentration of 15 mass % using carbon tetrachloride as a solvent. Theanalytical values are shown in Table 2.

Synthesis of Copolymer (B-2):

The synthesis is conducted in the same manner as in the copolymer B-1except that isoprene is used instead of 1,3-butadiene. The analyticalvalues are shown in Table 2.

Synthesis of Copolymers (B-3)-(B-7):

The synthesis is conducted in the same manner as in the copolymer B-2except that the charging ratio of monomers, amount of catalyst andhydrogen pressure are changed. The analytical values are shown in Table2. TABLE 2 Copolymer (B) B-1 B-2 B-3 B-4 B-5 B-6 B-7 Kind of SBR SIR SIRSIR SIR SIR SIR copolymer Bound styrene 50 50 48 50 22 50 45 content(mass %) Weight average 1.6 × 10⁴ 1.6 × 10⁴ 1.6 × 10⁴ 4.0 × 10³ 1.5 ×10⁴ 1.6 × 10⁴ 1.5 × 10⁵ molecular weight Hydrogenation 85 85 55 70 83 6590 ratio (%)

A rubber composition is prepared by mixing various components are mixedin a Banbury mixer according to a compounding recipe shown in Table 3,and vulcanization properties thereof are shown in Table 4. TABLE 3 partsby mass Copolymer (A) 100 Copolymer (B) variable Carbon black (ISAF) 100Aromatic oil 20 Stearic acid 2 Zinc white 3 Antioxidant *1 1Vulcanization accelerator *2 0.4 Vulcanization accelerator *3 1 Sulfur1.5*1 N-1,3-dimethyl-butyl-N′-phenyl-p-phenylenediamine*2 1,3-diphenyl guanidine*3 dibenzothiazyl disulfide

TABLE 4 Compar- Compar- Compar Conven- ative ative ative tional ExampleExample Example Example Example Example Example 1 1 2 3 2 3 Copolymer(A) A-1 A-1 A-1 A-1 A-1 A-1 A-1 (parts by mass) (100) (100) (100) (100)(100) (100) (100) Copolymer (B) B-1 B-2 B-3 B-4 B-5 B-6 B-7 (parts bymass) (80) (80) (80) (80) (80) (80) (80) Strength at break 100 105 104 99 103 104 105 (index) Wet-skid 100 110 101 104 100 106 105 resistance(index) Dry gripping 100 110 100 102  99 105 107 property (index) Wearresistance 100 105 103 100 104 104 104 (index) Compar- Compar- Compar-Compar- ative ative ative ative Example Example Example Example Example4 5 4 6 7 Copolymer (A) A-2 A-3 A-4 A-5 A-6 (parts by mass) (100) (100)(100) (100) (100) Copolymer (B) B-2 B-2 B-2 B-2 B-2 (parts by mass) (80)(80) (80) (80) (80) Strength at break  97  95 103  99  99 (index)Wet-skid 105 109 109 110 108 resistance (index) Dry gripping 104 111 110111 110 property (index) Wear resistance  99  94 102  98  96 (index)Compar- ative Example Example Example Example Example 8 5 6 7 8Copolymer (A) A-1 A-1 A-1 A-1 A-1 (parts by mass) (100) (100) (100)(100) (100) Copolymer (B) B-2 B-2 B-2 B-2 B-2 (parts by mass) (5) (20)(50) (100) (130) Strength at break 105 106 106 105 104 (index) Wet-skid 95 101 106 111 113 resistance (index) Dry gripping  97 102 108 112 112property (index) Wear resistance 107 106 105 103 102 (index)The property is separated by an index on the basis that ComparativeExample 1 is 100, in which the larger the index value, the better theproperty.

As seen from the results of Table 4, the vulcanizate of each example isexcellent in the wear resistance, fracture property, wet-skid resistanceand dry gripping property as compared with the vulcanizate of theconventional example using the hydrogenated styrene-butadiene copolymer.

Also, the vulcanizate of each example is excellent in the balance amongthe wear resistance, fracture property, wet-skid resistance and drygripping property as compared with the vulcanizates of ComparativeExamples 1-8 in which one or more of weight average molecular weight,bound styrene content and vinyl bond content in copolymer (A), weightaverage molecular weight, bound styrene content and hydrogenation ratioin copolymer (B), a relation between bound styrene content of copolymer(B) and bound styrene content of copolymer (A), and compounding amountsof copolymer (A) and copolymer (B) are outside the range defined in theinvention.

INDUSTRIAL APPLICABILITY

According to the invention, rubber compositions having good wearresistance and fracture property and capable of further improvingwet-skid resistance and dry gripping property are obtained by using thespecified styrene-butadiene copolymer (A) and the specified hydrogenatedlow-molecular weight styrene-isoprene copolymer (B) as a rubbercomponent and making the bound styrene content of the copolymer (B)larger by a given value than the bound styrene content of the copolymer(A). Such rubber compositions are suitable as a rubber for a tire tread.

1. A rubber composition comprising a rubber component formed bycompounding a styrene-butadiene copolymer (A) polymerized with a lithiumbased polymerization initiator, in which a polystyrene-conversion weightaverage molecular weight through a gel permeation chromatography is4.0×10⁵-3.0×10⁶ and a bound styrene content is 10-50 mass % and a vinylbond content in butadiene portion is 20-70%, with 10-200 parts by massof a hydrogenated styrene-isoprene copolymer (B) based on 100 parts bymass of the copolymer (A), in which a polystyrene-conversion weightaverage molecular weight through a gel permeation chromatography is5.0×10³-2.0×10⁵ and a bound styrene content is 25-70 mass % and not lessthan 60% of double bond in isoprene portion is hydrogenated, and havinga relation between the bound styrene content of the polymer (A) and thebound styrene content of the polymer (B) satisfying the followingequation:Bound styrene content of polymer (B)>bound styrene content of polymer(A)+10 (mass %).
 2. A rubber composition according to claim 1, whereinthe polystyrene-conversion weight average molecular weight of thecopolymer (A) is 7.0×10⁵-2.5×10⁶.
 3. A rubber composition according toclaim 1, wherein the bound styrene content of the copolymer (A) is 20-40mass %.
 4. A rubber composition according to claim 1, wherein the vinylbond content of the butadiene portion in the copolymer (A) is 30-60%. 5.A rubber composition according to claim 1, wherein not less than 80% ofdouble bond in the isoprene portion of the copolymer (B) ishydrogenated.
 6. A rubber composition according to claim 1, wherein thebound styrene content of the copolymer (A) and the bound styrene contentof the copolymer (B) satisfy a relation of the following equation:Bound styrene content of polymer (B)>bound styrene content of polymer(A)+15 (mass %).
 7. A rubber composition according to claim 1, whereinthe copolymer (B) is compounded in an amount of 20-100 parts by massbased on 100 parts by mass of the copolymer (A).
 8. A pneumatic tirecharacterized by using a rubber composition as claimed in claim 1 as atread rubber.
 9. A rubber composition according to claim 2, wherein thebound styrene content of the copolymer (A) is 20-40 mass %.